Susan Kotal's helpful article references testimonials, but applies equally well to asking clients to provide Google Reviews which are better for SEO (in my opinion). Testimonials on websites are valuable but they lack the stamp of independence of a review provided on a third party platform that is not under your control. Because a client has to provide a Google review independently through their own Google account, I think they carry a bit more weight despite the rumours of fake reviews and the like. One thing to note, if you happen to be active in the Yelp world, Yelp's terms of service forbid soliciting client reviews (yes, you read right).

Susan Kostal's four step approach provides a handy framework but I would modify her questions as follows

  1. What was your main goal when you hired our firm? 
  2. Did we help you achieve your goal?
  3. Did we exceed your expectations? If yes, how?
  4. What do you value most about your relationship with our firm?
  5. What was it like to work with us as a firm?
  6. If you were to refer our firm to a friend or family member, how would you describe us?
  7. Was there anything we can do better or that you would find helpful?

The last question is almost as valuable as a positive review because it provides data on how you can improve your client service and may well provide an avenue for maintaining future contact with past clients to enhance your referral network.